Evaluation of piezoelectric Ta2O5 thin films deposited on sapphire substrates

X-axis-oriented tantalum pentoxide (Ta₂O₅) piezoelectric thin films were deposited on sapphire (Al₂O₃) substrates using an RF magnetron sputtering system. The crystallinity and Rayleigh-type surface acoustic wave (R-SAW) propagation properties of the thin films were evaluated. For a Ta₂O₅ thin film deposited on the R-Al₂O₃ substrate plane, a similar preferential (200) orientation to that for a Ta₂O₅ thin film deposited on a SiO₂ glass substrate was obtained. For the first mode of the R-SAW on the R-plane Al₂O₃ sample, a coupling factor of 1.78% and a phase velocity of 4,895 m/s were obtained for a normalized film thickness of 0.225. For the c-Al₂O₃ substrate plane, the possibility of the epitaxial growth of a hexagonal Ta₂O₅ thin film with (203) orientation was shown. Furthermore, for the R-Al₂O₃ substrate plane, the possibility of the homoepitaxial growth of an orthorhombic Ta₂O₅ thin film with (201) orientation by using an oxide Ta thin film (TaO_x) as a buffer layer was also shown. However, no increase in coupling factor and no major improvement in propagation loss were observed upon these crystallizations because the hexagonal Ta₂O₅ has no piezoelectricity and the Ta₂O₅ thin film deposited on the TaO_x buffer layer had polycrystalline structure.